1. Field of the Invention
The present invention relates generally to a web particle removal method and apparatus, and more particularly to a web particle removal method and apparatus that is capable of precisely removing foreign matter, such as debris, dust, and the like adhering to a surface of a thermoplastic base material.
2. Description of Related Art
Photosensitive material, magnetic recording tape, film for optical use, and the like are manufactured by adding respective functions to a thermoplastic base material (which can only be called a xe2x80x9cwebxe2x80x9d) in order to express industrial values. Many of these products are manufactured by coating the surface of the thermoplastic base material with a photosensitive substance, a magnetic substance, a light-transforming substance, and the like. In recent years, an applying speed of these substances has been raised in order to improve the productivity. If foreign matter, such as debris, dust and the like adheres to the surface of the base material, a coating defect may be several hundred times larger than the size of the particle of the foreign matter. To solve this problem, a so-called particle removal process is executed to forcibly remove the foreign matter such as the particle adhering to the surface of the base material prior to the coating.
The foreign matter adheres to the surface of the base material for a variety of reasons as follows. Some foreign matter existing on the surface of a film is embedded in the surface before the film is solidified during the film formation. Some foreign matter adheres to the surface due to intermolecular force from the formation of a film to the coating of a functional material. Some foreign matter adheres to the surface due to electrostatic force generated by the electrification of a film surface. Some foreign matter adheres to the surface in such a manner that a liquid including a solid matter adheres to the surface and is then dried and solidified. The foreign matter has a variety of compositions and a variety of adhesions with respect to the film. The coating defect results from the foreign matter particle of submicron to several millimeters.
In a well-known dry process particle removal method, a non-woven fabric or a blade is pressed against the surface of a film as disclosed in Japanese Patent Provisional Publication No. 59-150571. In a method disclosed in Japanese Patent Provisional Publication No. 10-309553, the air with a high cleanliness is jetted at a high speed to exfoliate the deposits from the surface of the film and guide them into a near inlet. Shinko Ltd. commercially produces a xe2x80x9cNew Ultra Cleaner (trademark)xe2x80x9d that exfoliates and sucks the deposits by jetting the compressed air vibrated by ultrasonic waves. This cleaner is characterized in that a shearing stress of airflow is combined with the vibration by the ultrasonic waves in order to improve particle removing performance.
Japanese Patent Provisional Publication No. 10-290964 discloses another dry process particle removal method, wherein positive and negative air ions are implanted to neutralize electric charges to thereby remove the exfoliated foreign matter by another air flow.
There is also a wet process particle removal method. In an example of the wet process particle removal method, a film is guided into a cleaning liquid vessel to exfoliate the deposits by means of an ultrasonic generator. In another example of the wet process particle removal method, a cleaning liquid is supplied to a film, and the air is then jetted to the film at a high speed and the deposits are sucked as disclosed in Japanese Patent Publication No. 49-13020.
Although the above-mentioned particle removal methods are effective for the relatively-large deposits of dozens of micrometers or more or the deposits with a low adhesion, they are hardly effective for small deposits of several micrometers or less or deposits with a strong adhesion.
To address this problem, Japanese Patent Publication No. 5-50419 has proposed a method which comprises the steps of applying a solvent on the surface of the film and then pressing a rod member, which is rotating in a reverse direction to a film transporting direction, against the surface of the film to scrape off the deposits while the solvent is remaining. In this method, a small gap is formed between the film and the rod member in order to prevent the passage of deposits larger than the gap, and a shearing stress is transmitted through the solvent to exfoliate the deposits. Therefore, this method is effective for small deposits with a strong adhesion.
Japanese Patent Provisional Publication No. 62-65872 has proposed another particle removal method in that a blade with a sharp edge is provided instead of the rod member, which is rotating in a reverse direction to the film transport direction, to thereby improve a cleaning effect.
In the methods disclosed in Japanese Patent Publication No. 5-50419 and Japanese Patent Provisional Publication No. 62-65872, the rod member or the like is formed of a flat hard metal with an excellent abrasion resistance so that the rod member or the like can be in direct contact with the surface of the thermoplastic base material. For this reason, the film is damaged when a solid foreign matter enters the gap between the film and the rod member or the like. Moreover, when a liquid film between the film surface and the rod member or the like is broken, the film surface is damaged and new foreign matter is generated.
It is therefore an object of the present invention to provide a web particle removal method and apparatus that is able to remove foreign matter, such as debris, dust and the like adhering to the surface of a web-shaped roll without damaging it.
The above object can be accomplished by providing a web particle removal method, comprising the steps of: rubbing continuously a surface of a running web with an elastic body wet with a liquid; and after the rubbing step, spraying a liquid to the surface of the running web.
In the method of the present invention wherein the film surface is rubbed with the elastic body accompanied by the liquid, a shearing stress is thousands of times as large as that in a dry process particle removal method using a gaseous medium at the same shearing speed. Therefore, the particle removal method of the present invention can remove smaller foreign matter with a stronger adhesion compared with the dry process particle removal method. Since the elastic body has a lower rigidity than a metal rod and a blade, there is extremely little possibility that the elastic body pressed against the film damages the film surface.
The foreign matter that cannot be rubbed off by the shearing stress of the elastic body, and the foreign matter that adheres again to the film surface in company with the liquid, are washed off by spraying a liquid to the surface of the film at the downstream side of the elastic body.
The foreign matter, which has been rubbed off from the film surface, partially remains on the surface of the elastic body. This is accumulated as time passes, and adheres again to the film surface. This lowers a foreign matter removal ratio, and damages the film if the rigidity of the remaining foreign matter is higher than that of the film. To solve this problem, ultrasonic waves are applied to the surface of the elastic body in order to remove the transferred foreign matter after the rubbing of the film according to the present invention.